Photoionization-confined shell around a cool supergiant star. The second generation of low-mass stars are formed in the shell. This scenario could be common in the first few million years of the earlyglobular clusters, explaining the pollution of the second generation. This simple drawing serves to present the original idea; as for the nominal values of our model, the shell forms at r ≈ 0.02 pc from the central star (cf. our simulation of a shell in Fig. 7). The central supergiant itself has a stellar radius of ~5000 R⊙; that is, the supergiant is 170 times smaller in radial dimension than the sphere of the shell. (This figure is derived from Fig. 1 of Mackey et al. 2014.)
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